Planar fluorescent lamps are useful in many applications, including backlit displays and heads-up displays in aerospace applications.
Such lamps typically include a body having a chamber and one or more transparent faces from which light is emitted. Within the chamber, a gas containing mercury vapor produces ultraviolet energy in response to an electrical discharge provided by a spaced-apart pair of electrodes within the chamber. A fluorescent coating within the chamber converts the ultraviolet energy to visible energy, and the visible light is emitted through the transparent face to provide illumination. To extend the length across which the discharge will travel and thereby improve the efficiency of the lamp, an indirect path, such as a spiral or serpentine path, may be defined within the chamber by barrier walls.
While such lamps can provide excellent illumination during operation, they are often difficult to start and operate at low temperatures and/or low light levels. The principal cause of cold starting difficulty in low temperature environments is condensation of the mercury vapor within the lamp. In low temperature applications, the difficulty in starting lamps may be overcome to some extent by heating the lamp or by providing a heated environment in which the lamp is contained. Such an approach usually requires an external source of heat to be applied.
In low light and/or low temperature applications, uniformity of the discharge between the electrodes can be degraded, causing the light produced by the lamp to lack uniformity. For example, where the electrical excitation of the electrodes is insufficient to overcome cold starting conditions, it is difficult to generate a substantial discharge along the entire discharge path; consequently, dark areas can remain along the discharge path, causing the light emitted by the lamp to be uneven. Often, little or no light is emitted from the sections of the discharge path most distant from the electrodes.